[CMake] how to include a file that is under out-of-tree build directory

[Jun Sun]

I am staring to use cmake for an existing project.

We generate a config.cmake file under current build directory. We like
to include "config.cmake" in the in-tree CMakeLists.txt. cmake complains
about not able to find "config.cmake". 

Can someone help?

Here is the tree structure:

src/
 |
 +-- CMakeLists.txt ( include (config.cmake) )
 ...

build/
 |
 +-- config.cmake

Ceers.

Jun